1. Field of the Invention
The present invention relates generally to methods and apparatus for remotely measuring physical properties inside a sealed container without penetrating the walls of the container, and more particularly to a method for measuring the pressure, temperature, or other physical properties inside a non-magnetic, stainless steel storage container storing nuclear fuel and/or other hazardous products that can be implemented without requiring a power supply inside or outside of the container.
2. Description of the Prior Art
There are a variety of applications where the physical properties (such as pressure or temperature) of a substance, which may be either a gas or a liquid, inside a sealed storage container needs to be monitored or measured without any penetration of the external walls of the container and without any internal power supply. The storage of spent nuclear fuel or radioactive mixed waste is one such application. For safety reasons, the pressure inside such a container needs to be monitored to determine whether or not the container is likely to deform or rupture. Another application is the measurement of pressure or temperature inside a pressure vessel or pipe where electrical sensors cannot or are not easily or safely implemented or used.
There are a variety of different types and sizes of storage containers used by the United States Department of Energy (DOE) that need to be monitored for pressure. Conventional sensing systems that penetrate or tap into and through the walls of the container and affix a sensor to the inside of the container are avoided, because any penetration of the sealed container is a potential release site for the gas or liquid present in the container. These DOE storage containers are typically cylindrical in shape and are made of stainless steel. They can range from several inches in diameter and several feet in height to several feet in diameter and tens of feet in height.
It is important to determine whether or not any excessive buildup of pressure occurs in large, cylindrical, stainless-steel storage containers used to store spent nuclear fuel rods. Such containers at DOE""s Hanford Site are approximately 2 ft in diameter and 14 ft in height. A cold vacuum drying process is used to remove water from the container so that it can be sealed and welded closed prior to being taken to a dry-storage vault for a 40-year storage period. While the cold-drying process is very effective, traces of water may remain. Over time, radiolysis can decompose the traces of water into its component elements (i.e., hydrogen and oxygen). In addition, hydroxyl-containing materials will similarly decompose, contributing to the hydrogen-oxygen atmosphere forming in the sealed container. It is possible that the hydrogen-oxygen atmosphere could eventually reach concentrations where the pressure inside the container could deform or rupture the container.
The most direct method for determining if there is a pressure build-up inside a container is to visually inspect the container for physical deformation. The obvious problem with this approach is that the level of the pressure is unknown and may have already exceeded the safe limits of the container by the time the deformation is visually detectable.
In other applications, magnetic coupling systems have been used, including a sensor apparatus and a reading apparatus separated from each other by a non-metallic wall. Two examples are fluid flow meters and smart sensors for structures. However, none of these systems have been used for interrogating the inside of a sealed container, especially one containing nuclear waste materials, and none have been used where the distance between the sensing and readout elements is very large (e.g., many inches). U.S. Pat. No. 3,949,606 describes counting the rotations of a flow meter using permanent magnets mounted on rotational elements on both sides of a non-magnetic wall. Various configurations and strengths of magnets have been used to accurately count the number of rotations of a flow chamber. More recently, a variety of smart sensors have been disclosed for sensing the characteristics of a structure with strain and other such gauges (e.g., U.S. Pat. No. 5,433,115). These smart sensors include a sensing element and a magnetic coil embedded directly into the structure and an exciter magnetic coil and readout unit mounted on the outside of the structure that can interrogate the sensing elements through a gap made of a non-magnetic material.
In U.S. Pat. No. 5,150,115, an inductive-type rotary encoder is described that has a stator with a conductive pattern on one side and a rotor with a conductive pattern facing the stator, and apparatus for inductively coupling and reading the angular position of the stator relative to the rotor.
In U.S. Pat. No. 4,339,955, a method is described for measuring the displacement of a diaphragm inside a pressure gauge by reading the movement of a permanent magnet mounted on the displacement element with an inductive coil across a very small air gap. This invention avoids physical contact of the displacement element with the reading element.
In view of the prior art described above, it is apparent that there is a need for an apparatus that can measure physical properties inside a sealed container without penetrating the walls of the sealed container.
It is therefore an object of the present invention to provide a method and apparatus for measuring physical properties inside a sealed container.
It is another object of the present invention to provide a method and apparatus for measuring physical properties inside a sealed container that does not require penetrating the container walls.
It is a still further object of the present invention to provide a method and apparatus for measuring physical properties inside a sealed container that does not require internal electrical power.
It is another object of the present invention to provide a method and apparatus for measuring liquid or gas pressure inside a sealed container using permanent magnets and not requiring internal electrical power.
It is an object of the present invention to provide a method and apparatus for measuring physical properties inside a sealed container that uses internal and/or external power.
Briefly, a preferred embodiment of the present invention includes a method and apparatus for measuring gas pressure inside a sealed magnetic container. A transducer inside the container is responsive to pressure, having a Bourdon tube connected to a first pivotally mounted permanent magnet. The Bourdon tube has an oval cross section and a tube axis following a curved contour. A proximal end of the tube is open to the container atmosphere, and a closed distal end of the tube is coupled to the first magnet by means of a geared mechanism. As the pressure in the tube increases, the curved contour tends to straighten, causing the first magnet to rotate. A second permanent magnet is pivotally mounted and positioned exterior to the container, with the first and second magnets having a common pivot axis. The second magnet is rigidly attached to a pointer for indicating a pressure value indicated on a calibrated scale.
An advantage of the present invention is that it provides a safe method of measuring pressure in a nuclear waste container.
Another advantage of the present invention is that it provides a safe method of measuring a physical property inside a sealed container without penetrating the walls of the container.
A further advantage of the present invention is that it provides a method of measuring pressure in a sealed container without requiring electrical power.